There was no doubt that Aizen knew Silas was in the Soul Society. Otherwise, the gates wouldn't have been lowered from the get-go.1
From what Silas remembered, the Soul Society arc was one of the best arcs in all of Bleach. Most fans of the series favoured it. Ichigo and his friends engaged in relentless battles, each of them animated quite well. From the time Ichigo entered the Soul Society to even after he left, Aizen had manipulated almost every single one of those battles.
Technically, the first person Ichigo faced was Gin, who knocked him back into the Rukongai after he defeated Jidanbō. The only way for his friends and him to enter the Seireitei afterwards was to use a special cannon from the Shiba family, of which Isshin Kurosaki was the former head. This same family aided him successfully.
Upon entering the Seireitei, he then faced Ikkaku Madarame, a bald Soul Reaper from squad thirteen who also possessed a Bankai. His Bankai wasn't used during their battle because he believed he needed to hide it in order to continue fighting under his captain. Ichigo, not being aware of this, managed to defeat him. But his many encounters with other Soul Reapers wouldn't end there.
After defeating Ikakku, he would go on to have a battle with the lieutenant of squad eleven, Renji Abarai. Their fight proved formidable, with Ichigo delivering the final slash. He would eventually be healed by a Soul Reaper from squad four, the medical division, Hanatarō Yamada. Hanatarō, who also desired to save Rukia, decided to help Ichigo and his friends.
Once he was healed, his next encounter would be with the captain of squad twelve, Kenpachi Zaraki. This fight was heavily criticized on account that Ichigo, who still hadn't awakened his Bankai yet, had managed to defeat the captain. It was later explained how Zaraki's skills worked. He was the type to grow stronger based on his opponent's spiritual pressure. If his opponents were weak, he would lower his spiritual pressures to theirs for a fun fight. And if his opponents were stronger, his own spiritual pressure would increase until it matched or surpassed theirs. He was truly a scary opponent, someone who even Aizen was terrified to clash swords with!
Hundreds of years of fighting weak opponents dulled Zaraki's spiritual pressure to a fairly low point. The only reason Ichigo had won that fight was because Zangetsu suddenly raised his spiritual pressure, allowing Ichigo to overwhelm Zaraki before Zaraki's own spiritual pressure could rise.
The fights wouldn't end there. Yoruichi, who had been separated from Ichigo, managed to find him shortly after. She managed to heal his wounds but immediately left upon discovering where Rukia had been locked up. When he arrived on a bridge to save her, he was met by Byakuya. A full fight couldn't ensue due to Yoruichi's arrival, safely removing Ichigo from the vicinity to undergo his Bankai training.
Roughly three days were spent with him practicing with Zangetsu to achieve it. During that time, Renji made the ultimatum to also save Rukia by helping Ichigo and his friends. The latter managed to achieve Bankai one day prior. But he was later defeated by Byakuya's Bankai, Senbonzakura Kageyoshi.
Byakuya's Zanpakutō, named Senbonzakura, can separate into thousands of smaller, slender bladed fragments, leaving only the sword's handle in Byakuya's grip. While the blades are supposedly too small to be seen normally, they reflect light in such a way as to resemble cherry blossom petals. His Bankai, a much larger version of his Shikai, creates a row of a thousand giant blades to be separated, as opposed to just one sword!
Ichigo would fight against Byakuya in a match most anime enthusiasts were all too familiar with. During their epic clash, Ichigo's life was in peril, prompting the Hollow inside him to take over his body and decimate Byakuya. Since Ichigo wasn't aware of Zangetsu's true purpose, he fought back against his Hollowfication and managed to win the battle.
All of those events had been monitored by Aizen from the outside through the use of hidden cameras embedded in mechanical flies. How he was even able to get away with this was by killing Central 46, a courtroom of Souls that acted as the Soul Society's judiciary. Aizen's Zanpakutō's hypnosis abilities were just that powerful. Using Gin Ichimaru and another captain named Kaname Tōsen, the three acted as the new Central 46, who gave the order to further Rukia's execution date.
This was all for the purpose of acquiring what was inside Rukia, the Hōgyoku. What made this little device so powerful was that it could sense the hearts of those around it and materialize their deepest desires. Considering there are two Hōgyoku's, Aizen, who had created one, desired the second one in order to form a single Hōgyoku to help him attain power strong enough to rival the king of the Soul Society.
The moment Ichigo had saved Rukia, Aizen decided to go with his backup plan, which was to use one of Kisuke's devices that could safely separate objects from a Soul without complications. Extracting the Hōgyoku from Rukia proved to be smooth. Everything had gone according to his plans. Some captains managed to catch on to this scheme, such as Retsu Unohana, captain of squad four. Yet things were far too late.
Aizen could have obtained the Hōgyoku with far less complexity. Doing so, however, wouldn't have fulfilled his twisted desire to forge a worthy adversary capable of testing his Hōgyoku-enhanced powers. He meticulously orchestrated Ichigo's entire story, moulding his strength as a hybrid Soul Reaper, a crucible from which Aizen's own strength would be forged. However, Aizen's crafted game backfired. Little did he anticipate Ichigo's meteoric rise, his power surged beyond Aizen's wildest calculations—a fight that would be known in anime history as a rushed letdown since Kisuke was the true victor.
Not a single memorable event in Bleach was forgotten. Now that Silas was planning to take on the Soul Society without what Ichigo had, it was going to prove to be a challenge. Aizen's plans have undoubtedly changed. And though Silas's own actions within the Seireitei would not be so similar to Ichigo's, if he needed events to play out like he remembered, certain acts would have to be done.
Of those acts, most of them revolved around Rukia. If Aizen at least saw that "Ichigo" was still interested in saving her, certain Soul Reapers would be placed in the right positions for Silas to face.
1 If you already know Bleach's story, you're free to skip this chapter.